Apparatus for developing electrostatic latent images by liquid developing system



F b, 13, 1968 YUSHI MATSUMOTO ETAL 3,368,526

APPARATUS FOR DEVELOPING ELECTROSTATIC LATENT IMAGES BY LIQUID DEVELOPING SYSTEM Filed Nov. 28, 1966 2 Sheets-Sheet 1 INVENTOR M MBVOW 3,368,526 ELECTROSTATIC LATENT IMAGES VELOPING SYSTEM 2 Sheets-Sheet 2 HQ M QNWN N 8 a HMMHHI 0 WM W A INVENTORVS M E; W

Feb. 13, 1968 YUSHI MATSUMOTO ETAL APPARATUS FOR DEVELOPING BY LIQUID DE Filed Nov. 28, 1966 United States Patent 3,368,526 APPARATUS FOR DEVELGPING ELECTRO- STATIC LATENT IMAGES BY LIQUID DE- VELOPING SYSTEM Yushi Matsumoto, Tokyo, and Hirokazu Ono, Yokohama shi, lapan, assignors to Tokyo Shihaura Electric Co., Ltd., Kawasaki-sill, llapan, a corporation of Japan Filed Nov. 28, 1966, Ser. No. 597,276 Claims priority, application Japan, Dec. 1, 1965, ill/73,556 6 Claims. (Cl. li8637) ABSTRACT OF THE DISCLOSURE A liquid developing apparatus including an arcuate channel formed between a rotatable cylinder and a container with an electrode. In order to obtain a clear picture, the speed of a liquid developer in said channel is made to be substantially equal to that of rotation of said cylinder to form a latent image on a recording wheel carried by the peripheral surface of said cylinder. Thereafter squeeze means squeeze the recording sheet bearing the latent image to remove an excess liquid.

This invention relates to an apparatus for developing an electrostatic latent image formed on an electrostatic recording medium either by an electrophotographic method or by an electrostatic printing method, and more particularly to such an apparatus for the development of an electrostatic latent image by use of a liquid developer.

In liquid development, an electrostatic recording sheet upon which an electrostatic latent image has been formed by the electrophotographic method or electrostatic printing method is introduced into a body of the liquid developer prepared by dispersing fine solid particles in an electrically insulating liquid medium. The solid particles are caused to be attracted onto the electrostatic latent image areas on the recording sheet to develop the electrostatic image into a visible one. A developing device wherein removal of the developer remaining on the developing and recording sheet is almost automatically carried out is assembled as a structural unit with an electrophotographic apparatus or an electrostatic printing apparatus.

With a recording sheet developed with a conventional developing apparatus, the shape and the gradation of the darkness and blackness is an object to be reproduced can be reproduced with a high quality reproduction when the electrostatic latent image has a pattern of fine lines. When the electrostatic image forms solid area patterns, however, a high quality of tonal reproduction can be obtained only with respect to limited areas of the edge portions of the electrostatic image and the density becomes insuflicient successively toward the central portions thereof, there being a fear that a so-called edge effect is caused.

In order to form on the recording sheet a clearer visible image which follows the object, by reducing or removing the edge effect and by increasing the density of the developed image, an apparatus has been developed wherein development electrode plates are so disposed as to be close to the electrostatic image-bearing sheet and the liquid developer is supplied between the plate and the sheet. In order to sufiiciently improve the developed image in such an apparatus, it is necessary to arrange the recording sheet and the electrode plate as closely as possible within the small space between them but without touching them. This is difiicult since the spacing between one electrode plate and the other sheet-guiding electrode plate is set to have a narrow and uniform distance, of for example, about 1 mm. Further, when the electrostatic sheet passes through such a small spacing,

3,368,526 Patented Feb. 13, 1968 it tends to touch the electrode plate with the result that an image formed on the sheet and not yet fixed is damaged. The image so damaged 'will have undesirable thin stripe-like scratches.

An excess amount of developer remaining on the electrostatic sheet which has passed through a body of the developer and contains a developed image must be removed particularly from the image-bearing surface thereof to obtain a clear image. According to a prior art apparatus for liquid development, a pair of rollers having respectively a metallic and a rubber surface which are rotatable in opposite direction are provided in forced engagement relationship with each other at a position for taking up the recording sheet after submergence in the developer, and before the sheet is subjected to the following fixing process, the excess developer is squeezed between the two rollers while the image-bearing surface of the sheet advances in cont-act with the rubber roller. Such arrangement of the rollers is accompanied with certain defects in that solid particles constituting the unfixed image formed on the recording sheet are partly transferred onto the peripheral surface of the roller with the result that the density of the image after the sheet has passed the roller is rendered insufficient and also that the particles thus transferred onto the roller surface will be again deposited on the next succeeding recording sheet and thus spoil its image-carrying surface. Furthermore, when the metallic roller and the rubber roller having surface layers of considerably different elasticity are caused to rotate in forced engagement, their peripheral speeds become non-uniform and this will result in the distortion of the unfixed image on the recording sheet.

An object of this invention is to provide an apparatus for developing an electrostatic latent image by use of a liquid developer wherein the electrostatic latent imagebearing surface of an electrostatic recording sheet and a development electrode are confronted with a very small and uniform spacing thereby to sufiiciently remove an edge effect and to produce a high quality visible image.

A further object of this invention is to provide a developing apparatus wherein a liquid developer of constantly uniform composition is continuously supplied between the electrostatic latent image-bearing surface of an electrostatic recording sheet and a development electrode to flow in the same direction of, and at substantially the equal speed with, the travel of the recording sheet, so that a visible image having a sufiicient density and clearness may be produced at a favorable speed.

A still further object of this invention is to provide a developing apparatus wherein a pair of members rotatable in opposite directions in forced engagement relationship with each other are provided for the purpose of removing an excess amount of a liquid developer from a developed electrostatic recording sheet, one of said rotatable members which contacts the image-bearing surface of the recording sheet being a roller having an outer peripheral surface made of a layer of fibrous substance or an endless fibrous belt, whereby a clear and non-distorted image may be transferred to a next succeeding fixing aparatus.

These and other objects of this invention may be understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal cross section of a developing apparatus embodying this invention;

FIGS. 2A to 2C are side views of three modifications of a squeeze roller to be used in the apparatus of this invention; and

FIG 3 is a longitudinal cross section of a further modification of the squeeze roller.

Referring to FIG. 1, a developing apparatus comprises a developing means 11 for effecting development according to a liquid development system by submerging into a body of a liquid developer an electrostatic recording sheet bearing an electrostatic latent image thereon, a developer supplying means 12 including a vessel 13 and an associated supply system 14;, the vessel functioning to supply the liquid developer to said developing means and at the same time to receive the developer discharged therefrom, and a squeeze means 15 for removing an excess amount of the developer from the recording sheet which has been developed while being passed through the de veloping means 11.

The developing means 11 comprises a vessel 16 of conductive material such as metal, and a drum or cylinder 17 approximately the half of the entire outer spherical surface of which is kept submerged in the body of the liquid developer contained in the vessel 16, the vessel 16 serving to store the developer continuously charged from the developer supplying means and to discharge substantially such an amount of the developer as that supplied. The vessel has at one end, a storage portion 19 which is defined by a partition wall 2% and located below nozzle of an outlet pipe 18 of the supply system 14 to temporarily store the continuously supplied developer, and has its other end, a discharge portion 21 for overflowing and discharging the developer downwardly. The inner bottom surface of the vessel 16 is curved so as to face the peripheral surface of the cylinder 17 with an equal small spacing. The liquid developer is sucked from the storage container 13 into a suction pipe 22 of the supply system 14 by the action of a pump 23 and temporarily stored in the storage portion 19 of the vessel 16 after being fed therein from the nozzle of the outlet pipe 18. When the liquid level exceeds the height of the partition wall the developer is flowed through a narrow passage defined by the inner bottom surface of the vessel and the outer peripheral surface of the cylinder and returned to the container 13 from the discharge portion 21, thus being passed through a full circulation passage.

The cylinder 17 may be rotated in the direction of arrow by means of a suitable driving means (not shown), such as an electric motor and may have a diameter ranging, for example, from about 50 mm. to 200 mm. To the cylinder of the introducting side of an electrostatic recording sheet is press engaged a guide roller 24-. At the recording sheet take-out side of the cylinder 17 is disposed a squeeze means 15 for removing an ercess amount of the developer remaining on the recording sheet. Said squeeze means comprises a rotatable member 26 partly dipped in a body of liquid developer filled in a container 25 and which is rotatable in the direction of an arrow with the same peripheral speed as the cylinder 17 by means of a suitable driving source not shown, and another rotatable member 28 press engaging said rotatable member 26 having a fibrous surface thereon by the action of a spring 27 and rotatable with the same speed but in the opposite direction of rotation of the member 26.

The electrostatic recording sheet to which the liquid development system is applicable may be formed of a backing of paper or synthetic resinous material which is thinly coated with a photosensitive layer of photoconductive material in case application is made to the electrophotographic method, or with an insulating layer of electrically high resistance material in case application is made to the electrostatic printing method. In either case, the recording sheet will partly bear an electrostatic charge in response to the pattern to be printed so as to from an electrostatic latent image.

The recording sheet 29 hearing this electrostatic latent image is guided to the developing apparatus by suitable conveyer means and introduced between the cylinder 17 and the guide roller 24 press engaging thereto. Prior to the developing process, the storage container 13 is charged with a known liquid developer for electrostatic latent image development which is obtained by dispersing fine solid particles into a liquid medium of electric insulation, and the liquid developer thus charged is caused to pass through the vessel 16 along the above described circula tion passage and also through the container 25 of the squeeze means along a similar circulation passage not shown. With the developer flowing, the cylinder 17 and th rotatable members 26 and 28 are rotated with a predetermined rate of rotation so that their peripheral speeds are equal. A suitable amount of a DC. voltage may preferably be impressed upon the vessel 16.

The recording sheet 29 introduced between the cylinder 17 and the guide roller 24 adheres positively on the peripheral surface of the cylinder 17 due to the compressing force of the guide roller as well as the adhering force of the cylinder surface wetted while being rotated in the developer liquid, and successively advanced in the liquid developer flowing in the vessel 16. While passed in the developer in conformity with the rotation of the cylinder 17, the recording sheet 29 is subject to the pressure of the developer moving at a suitable speed, and is maintained at a position adhered on the cylinder surface.

The electrostatic latent image-bearing surface of the recording sheet, which comes in contact with the dcveloper in the vessel, will attract fine solid particles contained in the developer according to the magnitude of the electrostatic potential and form a visible image. This visible image formation is accelerated by the action of an electric field which is produced in the developer between the vessel and the electrostatic image. Since the electrostatic charge is induced to the bottom surface of the vessel 16 by the electrostatic charge on the recording sheet, the electric field is intensified and in the direction perpendicular to the surface of recording sheet, whereby also in the case of a relatively thick electrostatic latent image, solid particles in the developer are uniformly adhered on the entire surface of the image without being concentrically adhered onto the edge portions thereof, thus sufiiciently preventing an undesirable edge effect.

Such an advantage may be attained by arranging the spacing between the electrodes and the surface of the recording sheet to be about less than 2 mm., preferably about 0.5 mm., and the developing apparatus of this invention can satisfy this condition. According to this invention such electrodes are represented by the vessel 16 made of conductive materials for the liquid developer and the recording sheet may be passed through the developer, while being adhered on the peripheral surface of the cylinder closely spaced from the vessel 16.

According to the developing apparatus of this invention it is possible to move the developer in the same direction as that of travel of the recording sheet. If it is assumed that the cross sectional areas of the passage formed between the recording sheet 29 and the inner bottom surface for the vessel 16 are the same, the speed of flow of the developer is controlled by a difference in the liquid levels of the charging and discharging sides of the liquid developer. With the vessel wherein this difference in the liquid levels is selected to a desirable extent, the speed of flow of the developer can be substantially equal to that of the moving recording sheet. In this case, a successively advancing length of the recording sheet may always be in contact with a fresh developer, so that a visible image of a constant density may be formed over the entire surface of the recording sheet. Since the relative speed between the recording sheet and the developer is substantially zero, no lowering of clearness of the visible image which may be resulted from the fact that the solid particles once adhered on the electrostatic image are moved backwardly with respect to the direction of advance due to friction with the developer will not be caused.

Upon the electrode opposite to the surface of the recording sheet with a small spacing is impressed a bias voltage of the same or opposite polarity with respect to that of the electrostatic latent image formed on the recording sheet. A visible image of normal tone gradation or inverse one corresponding to those of the Original will beobtained by controlling the bias voltage, as known in the prior art, Further when the outer surface of the cylinder for supporting the recording sheet is made of a conductive material such as metal and impressed with a bias voltage of the opposite polarity as that of the electrostatic potential of the electrostic latent image, the solid particles are prevented from depositing on the non-image or discharged areas whereby the fog of the resulted image is reduced.

The recording sheet thus subjected to the developing operation through the developing means as above described may be transferred to a fixing means after an excess amount of the developer remaining thereon has been removed by a usual squeeze means. By the above described arrangement, it is possible to obtain a clear visible image which has sufficient density and excellent tone gradation wherein the edge effect is extremely reduced, when compared with one obtainable with a usual developing apparatus in the prior art. When removal of excess developer adhered on the recording sheet is carried out by passing the recording sheet through the squeeze means shown in FIG. 1, an unfixed visible image formed by passing through the developer will not in the least be damaged.

The squeeze means 15 shown in FIG. 1 has a pair of rotatable members 26 and 28 whose axes are arranged parallel with one rotatable member 28 being always press engaged with the other-rotatable member 26 by means of a coil spring 27. The rotatable member 26 has a surface layer 26' formed of porous fibrous substance and is so disposed that a part of it is dipped in a liquid developer filled in the container 25 located therebelow. Both of the rotatable members are caused to rotate at the same peripheral speed as that of the cylinder of the developing means 11 by means of a suitable driving source (not shown) connected with both or one of the rotatable members.

The recording sheet 29 which has been developed by passing through the liquid developer is separated from the surface of the cylinder 17 after submergence in the developer and moved upwards due to its stiffness to be squeezingly held between the rotatable members of the squeeze means 15. The visible image-bearing surface of the recording sheet is adapted to press contact the fibrous surface layer 26 of the rotatable member 26 by the rotatable member 28 which press contacts the opposite surface of said recording sheet and is held under the influence of the spring, whereby excess developer remaining thereon may be squeezed out without causing damage to the unfixed visible image. Although it is not fully understood by What particular mechanical structure damages to the visible image as experienced when using a rotary member having a rubber like surface layer can fully be prevented according to this invention wherein the surface layer of the rotatable member which contacts the visible image-bearing surface of the recording sheet is formed of fibrous material, it is believed this is due to the following reasons. The fibrous material has almost no adhesion and microscopically the surface has a suitable unevenness. Thus, it contacts the surface of the recording sheet not as a surface of relatively large area but as an extremely large number of points, whereby the visible image on the recording sheet is not subject to damages. For a similar reason, as solid particles electrostatically deposited on the recording sheet are not transferred to the rotatable member, there should be no fears of lowering of density of the visible image and contamination thereof which may be resulted when the particles once transferred onto the surface of the rotatable member as the latter rotates are again deposited on the surface of the succeeding recording sheet.

Among the fibrous materials which function with the above described effects may be those shown in FIGS. 2A to 2C other than a mass of natural or synthetic fibers bonded together by a suitable binder which is not soluble to the liquid developer. A rotatable member shown in FIG. 2A has a surface layer planted with a number of thin and short fibrous elements 52, the surface layer being knit fabric cloth wound in a desirable thickness. FIGS. 2B and 2C illustrate respectively a rotatable member having a surface layer formed of a knit fabric cloth layer 53 wound in a suitable thickness and cotton cloth 54 covering said layer 53, and a rotatable member having, as a surface layer, an oilproof sponge layer 55 coated with cotton cloth 56. Each of these surface layers may be used as fibrous endless belt capable of travelling among a plurality of rollers instead of being integral with the rotatable member.

FIG. 3 shows one example of the squeeze means wherein the surface layer of one of the two opposing rotatable bodies is a fibrous endless belt. A pair of rollers 61 and 62 are provided so as to oppose a rotatable member 28 pressedly held under the influence of a spring 27, said rollers 61 and 62 carrying an endless belt 63 of fibrous substance. One of the rollers 62 is pivotally supported by one end of an arm 65 which is movable about its fulcrum 64 disposed at the intermediate portion thereof, and is constantly pulled in the direction opposite to the other roller 61 by the action of a coil spring 66 connected to the other end of said arm. A part of or the entire portion of the roller 62 is dipped in a body of liquid developer contained in the developer storage container 13. Thus, the endless belt stretched between the rollers 61 and 62 is always stressed under a constant tension, and caused to successively submerge into the developer as the rollers rotate. Adjacent the outer peripheral surface of the roller er is disposed a roller 67 through the medium of the endless belt 63, said roller 67 being constantly pulled toward the roller 61 by a spring 68 to remove developer from the endless belt 63 which has been passed through the developer, and carried the same.

When the recording sheet reaches the position between the rotatable member 28 and the endless belt 63, its visible image-bearing surface contacts the endless belt 63 which has been suitably damped by the developer and the excess developer remaining on the surface is removed. Likewise in this case, the unfixed visible image will not be damaged and its distinction will not be lost due to undesirable contamination.

In the arrangement herein described, the squeeze means has been described as comprising two rotatable members 26 and 28. However, one of the rotatable members 28 which functions to urge the recording sheet 29 at its back side against the fibrous layer may be omitted and the cylinder 17 may be commonly used also for this purpose. in this case, the rotatable member 26 shown in FIG. 1 or the rotatable member 61 or the endless belt 63 in FIG. 3 will directly be press engaged by the drum 1'7 and rotated at the same circumferential speed therewith.

The recording sheet that has been treated with the squeeze means shown in FIG. 1 or 3 may be transferred to and fixed at the next succeeding fixing means (not shown). The resulted image will be the one having sufiicient darkness and excellent tone graduation wherein the edge effect is fully reduced.

While the invention has been disclosed with reference to the particular structure herein shown, it is not confined to the details and arrangement set forth, and this application is intended to cover such modifications or departures as may come within the scope of the appended claims.

What is claimed is:

1. A liquid developing apparatus for electrostatic latent images comprising: a container having a peripheral bottom of electro-conductive material;

a cylinder rotatably mounted in said container with its peripheral surface disposed adjacent to the bottom of said container to form a channel therebetween, so that an electrostatic recording sheet can pass through said channel in contact with said surface;

supply means for feeding a flow of liquid developer into said container in such a manner that the how speed of the liquid developer in said channel is substantially equal to the travelling speed of the peripheral surface of said cylinder;

a guide roller for pressing the passing recording sheet to the surface of the cylinder;

squeeze means including a rotatable member having a fibrous peripheral surface for squeezing the excess liquid developer which has adhered to the electro static recording sheet, by pressing the fibrous peripheral surface to the sheet surface, said rotatable member being in contact with the face of the recording sheet having a latent image thereon; and

means for continuously wetting the fibrous peripheral surface of said rotatable member with the liquid developer.

2. An apparatus as claimed in claim 1 wherein said squeeze means further includes a separate rotatable member disposed opposite-t and press-engaged with said rotatable member having the fibrous peripheral surface.

3. An apparatus as claimed in claim 1 wherein said rotatable member is an endless belt of fibrous substance.

4. An apparatus as claimed in claim 1 wherein said wetting means is a liquid developer having the same properties as said liquid developer fed into said container and a vessel for storing said wetting means, said rotatable member being partially clipped in the wetting means in said vessel.

5. An apparatus as claimed in claim 4 wherein said container includes a developer charging unit and a developer discharging unit spaced apart with respect to the diameter of said cylinder, said charging unit and said discharging unit being located at positions capable of moving the liquid developer through the spacing between said cylinder and the curved bottom of said container from the charging unit toward the discharging unit and connecting to means for continuously supplying the liquid developer to said charging unit and means for continuously discharging the liquid developer from said discharging unit.

6. An apparatus as claimed in claim I. wherein said supply means consists of a vessel for storing a liquid developer and a supply system for feeding the liquid developer from said vessel into the container.

References Cited UNITED STATES PATENTS 3,038,073 6/1962 Johnson 117-3'7 X 3,049,441 8/1962 Huang 11737 X 3,102,045 8/1963 Metcalfe et al 11737 X 3,169,887 2/1965 York 118-637 3,193,410 7/1965 Forrest et al. 1510() X 3,196,832 7/1965 Zin 118-637 3,276,896 10/1966 Fisher.

3,284,224 11/1966 Lehmann 118-637 X 3,299,787 1/1967 Kolb et al 118637 X CHARLES A. WILLMUTH, Primary Examiner.

P. FELDMAN, Assistant Examiner. 

